This invention relates to a semiconductor laser using a compound semiconductor material and more particularly to a semiconductor laser using a gallium nitride series material.
Recently, gallium nitride series compound semiconductor materials such as GaN, InGaN, AlGaN and InAlGaN have received much attention as blue semiconductor laser materials. The semiconductor laser using the above material can generate a fine light beam by reducing the wavelength thereof and is expected as a light source for high density information processing in an optical disk or the like.
As a semiconductor laser of short wavelength, a light source of 600 nm band using an InGaAlP material is already put into practice and the characteristics thereof are improved such that data can be read out from and written into the optical disk. A blue semiconductor laser is actively developed to further enhance the recording density and the oscillation operation of a semiconductor laser using a II-VI group series material is confirmed. Further, a GaN series semiconductor laser can generate a beam of short wavelength which can be reduced to 350 nm or less, the continuous oscillation at room temperatures for more than several thousand hours is confirmed and the reliability thereof is high, and the reliability of more than ten thousand hours is confirmed in an LED, and thus the semiconductor laser is actively studied and developed.
However, in this type of semiconductor laser, since the threshold current density cannot be made lower than 1 kA/cm.sup.2 and a far field pattern (which is hereinafter referred to as FFP) at the time of laser oscillation exhibits a multi-modal characteristic, a characteristic suitable for the light source of the optical disk or the like cannot be obtained.
In the conventional gallium nitride series semiconductor laser, since GaN is used for forming a contact layer and AlGaN is used for forming a clad layer, it oscillates in an anti-index guided mode. In the oscillation of anti-index guided mode, since FFP exhibits the multi-modal characteristic and a guided wave loss is large, an oscillation threshold value is generally large. Further, in the anti-index guided mode, since the effective refractive index in the laser oscillation is smaller than that of the contact layer, light leaks into the contact layer and the optical confinement coefficient of the contact layer becomes larger and a peak corresponding to the optical confinement of the contact layer occurs in FFP. This is the reason why the multi-modal characteristic occurs in FFP, and therefore, it is necessary to make the clad layer thick in order to reduce the leakage of light into the contact layer. As a result, the operation voltage in the laser oscillation becomes higher and a large number of cracks occur due to a difference in the lattice constant in crystallinity, thereby lowering the reliability.
In the case of a semiconductor laser with ridge structure formed of a GaN series material on a sapphire substrate, a phenomenon that light leaking from the clad layer is guided or a so-called anti-index guided mode phenomenon occurs since a GaN layer for the electrode contact on the exterior of the AlGaN clad is transparent with respect to oscillation light. Therefore, the oscillation due to a high-order vertical transverse mode having a smaller threshold current than the fundamental transverse mode tends to occur, a problem of kink of a light output due to mode competition by the adjacent high-order mode or a problem that it becomes difficult to focus the light by use of the lens occurs, and thus it is difficult to obtain characteristics suitable for the light source which can be used for the optical disk system.
That is, in the semiconductor laser using the gallium nitride series compound semiconductor material, since the contact layer lying between the clad layer and the electrode or substrate is formed of GaN, the clad layer is formed of AlGaN, the effective refractive index is smaller than that of GaN and oscillation occurs in the anti-index guided mode, then FFP exhibits the multi-modal characteristic, the guided wave loss is large and the oscillation threshold value becomes larger. Further, in the anti-index guided mode, since the optical confinement coefficient of the contact layer becomes larger, leakage of light causes FFP to have the multi-modal characteristic. In order to prevent this, it is necessary to increase the Al composition of the clad layer or make the clad layer thick, but as the result, cracks occur, the reliability is lowered and the operation voltage becomes higher, thereby making it difficult to provide an element which is used for the optical disk and is operated on a low voltage with a small threshold value and in which FFP exhibits the multi-modal characteristic.
Further, in a case where FFP is formed to exhibit a unimodal characteristic by forming an optical absorption layer of InGaN on the exterior of the clad layer and taking a measure for preventing leakage of light into the contact layer, it is inevitable that it oscillates in the high-order mode when it oscillates in the anti-index guided mode. At this time, since the oscillation threshold value in the high-order mode is approximately equal to the smallest oscillation threshold value, a problem that the mode order changes occurs at the time of oscillation and the oscillation frequency changes if the mode order changes, thereby causing a problem that the focus is deviated when laser light is focused by use of the lens. As a result, it becomes difficult to use the laser for reading or writing of the CD, DVD or the like.
In nitride series semiconductor, a hexagonal system crystal which grows on sapphire is obtained as the best crystal at present. A sapphire substrate is transparent in a range of 300 nm to a visible ray range and the refractive index thereof is smaller than that of GaN. In the conventional red semiconductor laser, since light which is not confined in the clad layer is absorbed by the substrate or electrode metal, light leaks into a portion other than the active layer is not almost present and the anti-index guided mode does not occur. However, in the nitride series semiconductor laser, since the sapphire substrate is used as the substrate, light leaks into a portion other than the active layer, a phenomenon called an anti-index guided mode in which a mode rises occurs and two or more peaks which cause a problem when it is used for the optical disk or laser projector occur in the far field pattern.
Thus, in order to provide a blue semiconductor laser which is used for the optical disk or the like and is operated on a low voltage with a small threshold value and high reliability and in which FFP exhibits the unimodal characteristic, it is important to solve the trade-off problems concerning the thickness and composition of the AlGaN clad layer, but at present, a construction which satisfactorily solves the problems is not yet provided. Since a transparent GaN layer with large refractive index is formed on the exterior of the AlGaN clad to attain the electrode contact, the vertical transverse mode is unstable irrespective of the current restricted structure and it is extremely difficult to attain a preferable beam characteristic.